Project Summary (30 lines) Over two million adolescents are infected with HIV globally, the majority due to perinatal transmission. As HIV-positive adolescents, their choices ? such as inconsistent medical adherence and unprotected sex - have the potential to alter the HIV epidemic. South Africa, home to the largest population of HIV-infected adolescents, is also experiencing a second epidemic of violence. Together, these epidemics create a devastating cycle. Intimate partner violence (IPV) is widely viewed as a key driver of HIV transmission, but is only part of the story. Violence victimization (at home, in relationships, and in the community) may similarly increase behavioral risk. To better understand how violence influences HIV transmission risk, we will enroll 300 HIV-positive and 300 HIV-negative male adolescents (aged 15-19) in Soweto, South Africa. At baseline and follow-up, participants will complete comprehensive surveys and medical testing (STIs, viral load). During the observation year, they will complete bi-weekly mobile phone surveys, yielding ecological momentary assessment (EMA) data on how daily experiences of violence affect behaviors in real time. In-depth interviews will be used to help us to better- understand how these behaviors are enacted by adolescents in their day-to-day lives. The aims are as follows: Aim 1: To measure IPV perpetration by PHIV, and test whether it exceeds that of HIV-negative peers. Most interventions to prevent IPV focus on girls only, but boys are increasingly recognized as essential participants. In urban South Africa, up to 40% of adolescent boys report perpetrating physical or sexual IPV. There is no data on IPV perpetration by male PHIV ? the population with the greatest potential to transmit HIV during adolescence.. Aim 2: To estimate the impact of violence victimization on behaviors associated with onward HIV transmission in both PHIV and HIV-negative peers. Exposure to violence may heighten risk of onward transmission by 1) decreasing HIV treatment adherence necessary for viral suppression, 2) decreasing condom use, and 3) increasing perpetration of IPV. Using longitudinal data and EMA, we will be able to establish temporal order between violence victimization and these three outcomes. Aim 3: To highlight potential intervention points by identifying the most salient mediators linking violence to HIV transmission behaviors. We will use structural equation modelling to test potential mediators (e.g., depression, gender ideology) between multi-level violence and behavioral risk. Combined with the above aims, this body of evidence will guide us towards effective, developmentally-appropriate interventions that prevent onward HIV transmission.